Since optical waveguide device is suited for integration and the like as well as for low power consumption, an application of such optical waveguide to optical switch, optical modulator and the like is studying. In recent years, a need for variable optical attenuator increases with a progress of DWDM (Dense Wavelength Devision Multiplexing) as a means for making optical powers of respective wavelengths uniform in case of multiplexing wavelengths, or an optical component in an optical ADM (Add Drop Multiplexer) for selecting an arbitrary wavelength to insert and remove it in a transmission line.
FIG. 1 is a block diagram showing a constitutional example of an optical ADM using a variable optical attenuator. Such optical ADM is disposed in midstream of an optical transmission line involving a plurality of channels (for example, thirty-two channels). In midstream of the transmission line, a demultiplexer 301 is placed on its input side, a multiplexer 302 is placed on its output side, and signal processing sections the number of which corresponds to that of the channels are disposed between both the sides. A channel in a signal processing section is composed of a 1×2 optical switch 303, a variable optical attenuator 304, and a 2×1 optical switch 305. Only a constitution of a signal processing section of a single channel is illustrated herein, but the other channels each of which has the same constitution.
A constitution of the signal processing section of a single section shown in FIG. 1 will be described. The demultiplexer 301 demultiplexes a multiplexed optical signal input in every different wavelength, and each of them is delivered to signal processing sections of respective channels. The 1×2 optical switch 303 is connected to each output line of the demultiplexer 301, either of output terminals of which is a Drop terminal, and an input terminal of the variable optical attenuator 304 is connected to the other terminal of the demultiplexer 301. To the variable optical attenuator 304, either of input terminals of the 2×1 optical switch 305 is connected, while the other input terminal is used as an Add terminal. An input terminal of the multiplexer 302 is connected to an output terminal of the 2×1 optical switch 305.
The optical ADM shown in FIG. 1 is disposed in midstream of an optical transmission line laid down with a certain distance. A multiple optical signal to be input to the demultiplexer 301 is amplified in an optical amplifier (not shown), and then the amplified signal is demultiplexed by the demultiplexer 301. Each of the demultiplexed signals is dropped (taken out to the outside) in response to switching of the 1×2 optical switch 303, or it is sent to its output side (side of the variable optical attenuator 304) without dropping the same. With respect to the optical signal sent to the output side, an amount of optical attenuation thereof is adjusted by the variable optical attenuator 304 in order to match an output level thereof with that of each channel. An optical signal from each of variable optical attenuators 304 is made to be multiple light by addition (multiplexing) in the multiplexer 302, and the resulting multiple light is output to the subsequent stage. Furthermore, when the 2×1 optical switch 305 was switched to an Add side, optical information taken from the Add end is input to the 2×1 optical switch 305, and it is added (multiplexed) to the multiple optical signal taken from the demultiplexer 301.
As the variable optical attenuator 304, the one having a structure composed of two directional couplers and two phase shifters disposed between the directional couplers wherein each of them has a directional coupler type Mach-Zehnder structure in which an optical waveguide is provided on a LiNbO3 (lithium niobate: LN) substrate being advantageous for downsizing and low electric power consumption is coming into practical use. By means of the variable optical attenuator 304 having such directional coupler type Mach-Zehnder structure, an electric field is applied to an optical waveguide through which optical signal passes to change a refractive index of a substrate, whereby an amount of attenuation in signal light can be controlled.
FIG. 2 is a perspective view showing a structure of a variable optical attenuator as a conventional optical waveguide device wherein a variable optical attenuator 200 is composed of a lithium niobate (LiNbO3) substrate (hereinafter referred to as “LN substrate”) 1, an electrode 2 formed on the LN substrate 1, and a SiO2 film 3 placed in between the electrode 2 and the LN substrate 1. Moreover, optical waveguides 4a and 4b are disposed in the vicinity of a surface of the LN substrate 1 on the opposite sides of the electrode 2. The electrode 2 has a three-layered structure of an ITO (indium oxide to which tin has been added: Indium Tin Oxide) thin film 21, a titanium (Ti) thin film 22 disposed on the ITO thin film 21, and a gold (Au) thin film 23 disposed on the titanium thin film 22. A voltage having + polarity is applied to the electrode 2, while a voltage having − polarity is applied to another electrode (not shown).
The ITO thin film 21 is made of indium oxide to which has been added tin, and it is a transparent electrode having 90% or higher transmittance of visible light and 10Ω/□ or less sheet resistance value. In the ITO thin film 21, it is prevented from an increase in insertion loss as a result of appearing optical absorption by approaching the titanium thin film 22 and the gold thin film 23 to optical waveguides 4a and 4b through the SiO2 film due to displacement (displacement in positions of the electrode 2 and the optical waveguides 4a and 4b). Furthermore, the titanium thin film 22 functions as an adhesive for bonding the ITO thin film 21 and the gold thin film 23 to each other. The gold thin film 23 functions as an electrode plate used for connection with the outside, and it is selected on the basis of such reasons that gold is excellent in adhesive properties as a result of alloying the same and that gold makes easily wire bonding.
According to a conventional optical waveguide device, however, such fact as described hereunder has been found. Namely, when such a situation that a voltage is applied under a specific atmosphere, for example, a high-temperature atmosphere (e.g., +80° C.) continues, an oxide of the titanium thin film 22 exhibits alkaline, so that it reacts with indium oxide of the ITO thin film 21 to produce ion flux, whereby the ITO thin film 21 is gradually solved out, and it results finally in electrode short-circuit. Because of an appearance of such electrode short-circuit; reliability and expected long life in an optical waveguide device decrease remarkably.